Correction of high grade carotid artery stenosis may result in cerebral hyperperfusion because of defective vascular autoregulation. Thus, transcranial Doppler was used to determine mean arterial flow velocity (Vmean) of the middle cerebral artery in 95 patients before and after carotid endarterectomy. Attention was focused on postoperative episodes of ipsilateral headache and hypertension. Symptoms of cerebral hyperperfusion lasted for 3 (1.5-5) h (median and range) in nine patients, and for 12 (8-14) days in nine other patients. Of these later patients, two developed seizures on the 5th and 6th postoperative day, respectively. The mean pressure difference across the stenosis was 31 (0-63) mmHg in the symptomatic group (n = 18) as opposed to only 10 (0-60) mmHg in the asymptomatic group (n = 77) (p < 0.01). In the 18 patients with headache after surgery, ipsilateral Vmean increased to 177 (130-332)% of the preoperative value (p < 0.0001), while the contralateral Vmean remained unchanged. After blood pressure was reduced in symptomatic patients with labetalol, ipsilateral Vmean decreased from 92 (69-124) to 56 (32-93) cm s-1 (p < 0.0001) as systemic arterial pressure decreased from 101 (80-128) to 88 (60-103) mmHg, with no change in contralateral Vmean. Normalisation of Vmean via reduction of arterial pressure ended episodes of headache and seizure in symptomatic patients. Thus, in patients who developed post-endarterectomy hyperperfusion, these findings clearly demonstrated that ipsilateral middle cerebral artery mean flow velocity was pressure dependent. This substantiates the hypothesis of defective autoregulation in the ipsilateral hemisphere after carotid endarterectomy, and in turn demonstrates an immediate cessation of symptoms with reduction of arterial pressure even in normotensive patients.